1. Field of the Invention
The present invention relates to a field sequential liquid crystal display (FS-LCD), and more particularly, to an LCD capable of obtaining desired chromaticity and luminance regardless of a driving current distribution of a light emitting diode (LED).
2. Description of Related Art
A color LCD generally includes a liquid crystal panel having an upper substrate, a lower substrate, and a liquid crystal injected between the upper and lower substrates. The color LCD further includes a driving circuit for driving the liquid crystal panel, and a back-light for providing white light to the liquid crystal. Such an LCD may be mainly classified into a red (R), green (G), blue (B) color filter type or a color field sequential driving type depending on its driving mechanism.
In the color filter type LCD, a single pixel is divided into R, G, and B subpixels, and R, G, and B color filters are respectively arranged in the R, G, and B subpixels. Light is transmitted from a single back-light to the R, G, and B color filters through the liquid crystal allowing a color image to be displayed.
On the other hand, a color FS-LCD includes, R, G, and B back-lights that are arranged in a single pixel that is not divided into R, G, and B subpixels. The light of the three primary colors is provided from the R, G, and B back-lights to the single pixel through the liquid crystal so that each of the three primary colors are sequentially displayed in a time-sharing, multiplexed manner, allowing the display of a color image using a residual image effect.
FIG. 1 is a perspective view of a configuration of a typical color FS-LCD.
Referring to FIG. 1, the FS-LCD includes a liquid crystal panel 100 having a lower substrate 101 in which a thin film transistor (TFT) array (not shown) for switching is arranged to be connected to a plurality of gate lines, a plurality of data lines, and a plurality of common lines. The liquid crystal panel also includes an upper substrate 103 in which a common electrode (not shown) is formed to provide a common voltage to the common lines. The liquid crystal panel further includes a liquid crystal (not shown) injected between the upper and lower substrates.
The FS-LCD further includes a gate line driving circuit 110 for providing scan signals to the plurality of gate lines of the liquid crystal panel 100, a data line driving circuit 120 for providing R, G, and B data signals to the data lines, and a back-light system 130 for providing light corresponding to three primary colors, namely, R, G, and B colors, to the liquid crystal panel 100.
The back-light system 130 includes three back-lights 131, 133, and 135 respectively providing R, G, and B light, and a light guide plate 137 providing the R, G, and B light respectively emitted from the R, G, and B back-lights 131, 133, and 135, to the liquid crystal of the liquid crystal panel 100.
Typically, a time interval of a single frame driven at 60 Hz is 16.7 ms ( 1/60 s). When the single frame is divided into three subframes, as is the case for the FS-LCD, each subframe has a time interval of 5.56 ms ( 1/180 s). The time interval of one subframe is short enough to prevent its field change to be perceived by the human eye. Accordingly, the human eye sees the three subframes during the time interval of 16.7 ms as a single frame, resulting in the recognition of a composite color formed by the three primary colors to display the image.
In order to obtain fast operating characteristics of the LCD, the response speed of the liquid crystal should be fast and the corresponding switching speed for turning the R, G, and B back-lights on and off should also be relatively fast. In addition, in order to obtain good image quality of the LCD, light having uniform chromaticity and luminance should be emitted from each of the LEDs.
FIG. 2 is a schematic block diagram of a back-light driving circuit used in the FS-LCD shown in FIG. 1.
Referring to FIG. 2, the back-light 220 includes R, G, and B back-lights 221, 223, and 225 for sequentially emitting R, G, and B lights, respectively, per each subframe. A back-light driving circuit 210 includes a driving voltage generator 211 sequentially generating a driving voltage VLED for driving the R, G, and B back-lights 221, 223, and 225.
Among these back-lights 220, the R back-light 221 emitting the R light includes a red LED (RLED), and the G back-light 223 emitting the G light includes a green LED (GLED), and the B back-light 225 emitting the B light includes a blue LED (BLED).
The driving voltage generator 211 generates the driving voltage VLED of a same level to the R, G, and B back-lights 221, 223, and 225. The driving voltage VLED provided from the driving voltage generator 211 is supplied to an anode electrode of the RLED of the R back-light 221, an anode electrode of the GLED of the G back-light 223, and an anode electrode of the BLED of the B back-light 225.
The back-light driving circuit 210 further includes a luminance adjuster 212 that is serially connected between the back-light 220 and a ground, and that adjusts the luminance of light emitted from the back-light 220. The luminance adjuster 212 has a first variable resistor RVR that is connected between the ground and a cathode electrode of the RLED of the R back-light 221 and that adjusts the luminance of light emitted from the R back-light 221, a second variable resistor GVR that is connected between the ground and a cathode electrode of the GLED of the G back-light 223 and that adjusts the luminance of light emitted from the G back-light 223, and a third variable resistor BVR that is connected between the ground and a cathode electrode of the BLED of the B back-light 225 and that adjusts the luminance of light emitted from the B back-light 225.
In the prior art, when a forward driving voltage VLED of, for example, 4V is sequentially supplied to the R, G, and B back-lights 221, 223, and 225, the variable resistors RVR, GVR, and BVR of the luminance adjuster 212 are used to sequentially provide a driving voltage suitable for the RLED, a driving voltage suitable for the GLED, and a driving voltage suitable for the BLED. Accordingly, a suitable forward driving voltage is supplied to each of the red, green, and blue LEDs per each subframe, so that the R, G, and B back-lights 221, 223, and 225 sequentially emit light having a desired luminance. That is, in the prior art, all of the R, G, and B back-lights 221, 223, 225 are provided with the same driving voltage, such as, 4V, so that the luminance of light emitted from the R back-light 221 is adjusted by applying a forward driving voltage RVf suitable for the RLED using the first variable resistor RVR when the RLED is required to be driven.
Meanwhile, the luminance of light emitted from the G back-light 223 is adjusted by applying a forward driving voltage GVf suitable for the GLED using the second variable resistor GVR when the GLED is required to be driven. In addition, the luminance of light emitted from the B back-light 225 is adjusted by applying a forward driving voltage BVf suitable for the BLED using the third variable resistor BVR when the BLED is required to be driven.
As mentioned above, the luminance was properly adjusted in the prior art by adjusting the variable resistor. However, LEDs forming the back-light generally have a very large distribution of driving currents based on the particular LED product. The differing driving currents from LED to LED create luminance non-uniformity that cannot be solved even when the luminance is adjusted using the variable resistor. Furthermore, chromaticity also cannot be adjusted due to the differing driving current distributions from LED to LED.